Why are people who are always smiling, happy, friendly and easy to tall to considered dumb?

Never heard that before

They aren’t

They might get underestimated every now and then maybe because they don't appear on the surface to have any depth.

I don't know that they are considered dumb in general. There might be a sprinkling of genuinely dumb ones in there...I just don't know the people who are judged that way.

I will tell you that in normal society, people don't discuss fine literature or physics, at least...not right away. 😉 They know that the grease that turns the wheels of casual conversation are comments about the weather, sports, favorite tv shows, and local restaurants.

Who said this ?

They are not. People who are considered "dumb" (if that is the correct word) are usually not very friendly or happy.

They aren’t considered so by me, or by others I whom know. I’m not familiar with that generalization. 😒